As many physical properties of objects can be measured accurately and non-contactly by optical measuring technologies, optical measuring methods are frequently used for measuring surface features in industries, such as wafer manufacturing, semi-conductor manufacturing, liquid crystal display manufacturing, electro-mechanical automation engineering, electro-optical measurements, and so on. Namely, optical measuring methods are adapted for measuring surface roughness and flatness of a wafer, for measuring bump sizes and coplanarity in a flip-chip manufacturing process, for measuring sizes of a space used for an LCD's color filter, or for measuring surface characteristics of a microstructure, etc.
There are many optical measurement techniques currently available, including phase-shifting interferometry, white-light interferometry, and confocal microscopy, etc. They are designed for different measurement conditions/environments and for different applications. The conventional phase-shifting interferometry is a technique about the interference of two light beams, i.e. an object beam and a reference beam, with different optical paths. According to the fringes interfered by the two light beams, the phase distribution of the object beam can be precisely calculated with different phase differences caused by different phase-shifts, then the phase information of the object beam can be precisely calculated. The principle of white-light interferometry is that: the maximal interference intensity occurs when the optical path difference for an object beam and a reference beam for all wavelengths is zero, so the plane with the zero optical-path difference can intersect the surface of an object to show the contours with the same height, then by vertically scanning, the height distribution of the object surface can be derived by combining many different equal-height contours. The confocal microscopy is an imaging technique used to measure a three-dimensional (3D) image by using a spatial pinhole to eliminate out-of-focus light. All the abovementioned methods are common in that: they all need to compare and analyze interference images acquired in sequence so as to reconstruct the 3D profile of an object. However, as those interference images acquired in sequence can be easily affected by vibration caused by surrounding environments, inaccurate measurements can be resulted if so.
U.S. Pat. No. 6,268,923 discloses an optical method and an optical system for measuring the 3D surface of an object. The optical method first splits a light beam emitted from a light source into three reference beams while varying the optical paths of the three reference light beam by manners such as shifting the position of a reference mirror in the optical system, varying the thickness of a glass plate used in the optical system, or changing the tilt angle of the reference mirror, and so on, so as to achieve the phase-shifting of the three reference beams simultaneously, and then the three reference beams are respectively directed to interfere with an object beam for enabling three different image sensors to acquire three interferograms simultaneously.
U.S. Publ. No. 2001/0035961 discloses a shape measuring apparatus, by which a reference beam and an object beam are directed to a quarter-wave plate to form a left-circular polarized beam and a right-circular polarized beam respectively, and the two polarized beams are split respectively toward three polarizers with different polarizing directions so as to form three interference images with different phase shifts while the three interference fringe images are detected by a single image sensor simultaneously.
In addition, each of two U.S. Pat. Nos. 6,304,330 and 6,552,808 reveals a method and apparatus for splitting, imaging and measuring wavefronts with interferometry. Each of the two abovementioned U.S. patents directs a 45-degree linearly polarized reference beam and a 45-degree linearly polarized object beam to a quarter-wave plate so as to form respectively a left-circular polarized beam and a right-circular polarized beam. Then direct the polarized beams to a phase-shifting array for generating four interferograms with different phase shifts and then taking the images of the four interferograms by a single image sensor simultaneously.
Moreover, Germany Pat. No. 19,652,113 discloses an improved Michelson interferometry apparatus, by which three interferograms with different phase shifts can be captured by a single image sensor simultaneously. In the optical apparatus, a reference beam is diffractively split into three beams with the same polarization by a grating, including the −1th-order, 0th-order, and 1th-order diffraction beams, in which the three diffraction beams are further directed to pass through a quarter-wave plate to create different phase delays. Finally, the three beams are combined and pass through a polarizer to form three different phase shifts for creating three interferograms with an object beam.